Bipolar transistor

ABSTRACT

A bipolar transistor using a B-doped Si and Ge alloy for a base in which a Ge content in an emitter-base depletion region and in a base-collector depletion region is greater than a Ge content in a base layer. Diffusion of B from the base layer can be suppressed by making the Ge content in the emitter-base depletion region and in a base-collector depletion region on both sides of the base layer greater than the Ge content in the base layer since the diffusion coefficient of B in the SiGe layer is lowered as the Ge contents increases.

FIELD OF THE INVENTION

[0001] The present invention concerns a bipolar transistor having anSiGe alloy base layer and, more in particular, it relates to a bipolartransistor suitable to a ultra-high-speed digital IC and microwave ormillimeter wave wireless communication IC, as well as a communicationsystem in which the bipolar transistor is applied.

BACKGROUND OF THE INVENTION

[0002] Prior art related to a bipolar transistor using B-doped SiGealloy (SiGe alloy) for a base is described in Technical Digest ofInternational Electron Devices Meeting (IEDM), 1997 (pages 791-794). TheSiGe base bipolar transistor of the prior art is to be explained withreference to FIG. 17(a) and FIG. 17(b). Graphs in the drawing showtypical two examples of a depth profile of impurity concentrations andGe contents in an active region of a SiGe base bipolar transistor of theprior art (refer to FIG. 4 showing a cross section for the main portionof a transistor; along broken line A) of the prior art. In the graphs, aregion of a shallow depth is for an emitter and a portion with a largedepth is for a collector. In the example shown in FIG. 17(a), the Gecontent increases at a constant grade toward the collector in a regionof a base layer in which B is doped, reaches a maximum value at an edgeof the base layer, and is kept at the maximum value in a region of apredetermined width in a base-collector depletion layer. In the exampleshown by FIG. 17(b), the Ge content has a constant value in a region ofthe base layer in which B is doped and in a region of a predeterminedwidth in the base-collector depletion layer.

[0003] For improving the operation speed of a bipolar transistor, it isnecessary to shorten a base transit time of carriers and lower a baseresistance. For this purpose, it is necessary for the depth profile of Bin the base layer to make the width as narrow as possible and increasethe concentration as high as possible. However, in the prior artdescribed above, since B in the base layer is diffused by a heattreatment after forming the base layer, it results in a problem that thedepth profile becomes broad to widen the width. In addition, since thebroadening for the width of the profile due to diffusion increases asthe concentration of B is higher, it was difficult to make therestriction for the width of profile and increase of the concentrationcompatible with each other.

[0004] Another problem in the prior art transistor is that an effectivebase width is widened to lower the operation speed if the collectorcurrent is increased. This is caused by injection of holes forneutralizing the negative charges due to increase of a collector currentdensity in a region at low impurity concentration, so that an energyband structure for a base-collector junction is changed as shown in FIG.18.

[0005]FIG. 15 shows dependence of a diffusion coefficient of B in anSiGe alloy on a Ge content. As the Ge content increases, the B diffusioncoefficient decreases to lower the diffusion speed. The problem ofincreasing the depth of B can be solved by utilizing this phenomena byadopting the following means.

[0006] If the Ge content is increased in a region adjacent to theB-doped layer of the base, that is, in an emitter-base depletion regionor a base-collector depletion region, since the diffusion speed of B inthe portion is lowered, widening of the width in the depth profile of Bcan be suppressed.

[0007] When taking notice only on the decrease of B diffusion, the totalGe contents may be increased in the emitter-base junction depletionregion, the base layer, and the base-collector depletion region, but thetotal amount of Ge in the SiGe alloy layer is excessively increased.Therefore, this causes strong stresses in the SiGe alloy layer due tothe difference of covalent bonding radius between Si and Ge, to cause aside-effect of forming crystal defects.

[0008] The side effect can be suppressed by controlling the Ge contenthigher in a portion adjacent to the B-doped layer of the base and lowerin the B-doped layer as much as possible. Since the total Ge content canbe decreased by this structure, occurrence of crystal defects can besuppressed. In this case, broadening of the distribution of B caused bythe reduction of the Ge content in the B-doped layer is almostnegligible. This is because the distribution of B is generally uniformin the B-doped layer, so that B is less diffused and the degree of thediffusion coefficient in this region gives less effect on the broadeningof the depth profile of B. For lowering the Ge content in the B-dopedlayer and kept it high in the portion adjacent therewith therebydecreasing the total Ge content as low as possible, the Ge concentrationmay be changed abruptly as much as possible at both ends of the B dopedlayer.

[0009] Further, the effect of suppressing the broadening of the Bprofile compared with the prior art can be attained also by making theGe content in the emitter-base junction at least equal with the contentin the B-doped layer though it is not higher than the content in theB-doped layer, by the same reason as described above.

[0010] Further, the problem that the effective base width is increasedby the increase of the collector current can also be improved by themethod described above of changing the Ge concentration as sharp aspossible at the edge of the B-doped layer on the side of the collector.This reason is to be explained below. In a transistor of the prior art,an effective base width is increased as the collector current densityincreases as shown in FIG. 18 since the grade of the band become lesssteep at the edge of the base on the side of the collector. On the otherhand, when the Ge concentration is changed abruptly at the edge of theB-doped layer, a nodge is formed in a valance band at a position atwhich the Ge concentration changes as shown in FIG. 2. The reverseV-shaped nodge on the side of the collector has a function of hinderingthe effect of smoothing the grade of the band in that portion when thecollector current density increases. This is because a great amount ofholes are accumulated at that portion if the grade is reduced andelectrical neutral can no more be kept in the vicinity thereof. Theeffect of suppressing the change of the gradient of the band withincrease of the collector current density can improve the problem oflowering the operation speed in a case of increasing the collectorcurrent.

SUMMARY OF THE INVENTION

[0011] Based on the considerations as described above, the presentinventor provides a bipolar transistor of the following structure and acommunication system by applying such a bipolar transistor.

[0012] At first, a bipolar transistor using a B-doped Si and Ge alloy(SiGe) according to the present inventor has a basic feature in that themaximum value of Ge content in an emitter-base junction depletion regionand a base-collector junction depletion region is greater than anaverage value in the base layer. In this bipolar transistor, it ispreferred that the grade of Ge content in a region in which the Gecontent is increased from the vicinity of the edge of the base on theside of a collector to the collector is made greater than the averagegrade of Ge content in the base layer.

[0013] Further, another feature of the present invention resides in abipolar transistor using a B-doped SiGe alloy (SiGe) for a base in whichthe maximum value of the Ge content in the base-collector junctiondepletion region is set greater than the average value in the baselayer, wherein the grade of Ge content in the region in which the Gecontent is increased from the vicinity of the edge of the base on theside of a collector to the collector is made greater than the averagegrade of the Ge content in the base layer.

[0014] In a further aspect of a bipolar transistor according to thepresent invention, a bipolar transistor using a B-doped Si and Ge alloy(SiGe) for a base in which the maximum value for the Ge content in thebase-collector depletion region is set greater than an average value inthe base layer has a region in which the Ge content is constant from theedge of the base layer on the side of the emitter to the emitter-basejunction. In this structure, it is preferred that the grade of Gecontent in the region in which the content increases from the vicinityof the edge of the base layer to the collector is made greater than theaverage grade of Ge content in the base layer.

[0015] Further, in an optical transmission system according to thepresent invention comprising

[0016] an optical receiver system having an a photodetector forreceiving an optical signal and outputting an electric signal, a firstamplifier for receiving the electric signal from the photodetector, asecond amplifier for receiving the output from the first amplifier, adecision circuit for converting the output from the second amplifierinto a digital signal in synchronization with a predetermined clocksignal, and a circuit for separating and converging the digital signal,and

[0017] an optical transmitter system having a circuit for synthesizingmultiple digital signals, a semiconductor laser and a semiconductorlaser driver for driving the laser,

[0018] at least one of transistors in the first and the secondamplifiers, the decision circuit, a multiplexer for digital signals, amultiplexer for multiple digital signals and the semiconductor laserdriver is constituted with the SiGe base bipolar transistor as definedabove in either one of the optical receiver system and the opticaltransmitter system.

[0019] Further, in a millimeter wave transmission system according tothe present invention having a receiving antenna for millimeter wave(frequency: 30 GHz-300 GHz), a first amplifier for amplifying a receivedelectric signal from the antenna, a receiving mixer for receiving theoutput from the first amplifier and stepping-down the frequency, a firstoscillator, a transmitting mixer for receiving a transmission electricsignal and stepping-up the frequency, a second oscillator, and a secondamplifier for receiving the output of the transmission mixer andamplifying the power,

[0020] at least one of the transistors in the first and the secondamplifiers, the first and the second oscillators and the transmissionmixer is constituted with the SiGe base bipolar transistor as definedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor using a SiGealloy for a base in the first embodiment according to the presentinvention;

[0022]FIG. 2 shows an energy band structure in an active region of an Sibipolar transistor in the first embodiment according to the presentinvention;

[0023]FIG. 3 shows a cross sectional structure of a bipolar transistorin the first to the seventh embodiments according to the presentinvention;

[0024]FIG. 4 shows a cross sectional structure for a main portion of abipolar transistor in the first to the seventh embodiments according tothe present invention;

[0025]FIG. 5 shows a cross sectional structure for a main portion ofmain steps in a method of manufacturing a bipolar transistor in thefirst to the seventh embodiments according to the present invention;

[0026]FIG. 6 shows a cross sectional structure for a main portion inmain steps of a method of manufacturing a bipolar transistor in thefirst to the seventh embodiments according to the present invention;

[0027]FIG. 7 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor in the secondembodiment according to the present invention;

[0028]FIG. 8 is a graph showing a depth profile of impurityconcentration with and Ge contents of a bipolar transistor in the thirdembodiment according to the present invention;

[0029]FIG. 9 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor in the fourthembodiment according to the present invention;

[0030]FIG. 10 shows an energy band structure in an active region of abipolar transistor in the fourth embodiment according to the presentinvention;

[0031]FIG. 11 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor in the fifthembodiment according to the present invention;

[0032]FIG. 12 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor in the sixthembodiment according to the present invention;

[0033]FIG. 13 shows an energy band structure in an active region of abipolar transistor in the sixth embodiment according to the presentinvention;

[0034]FIG. 14 is a graph showing a depth profile of impurityconcentrations and Ge contents of a bipolar transistor in the seventhembodiment according to the present invention;

[0035]FIG. 15 is a graph showing a relation between a Ge content and a Bdiffusion coefficient in an SiGe alloy layer;

[0036]FIG. 16 is a graph comparing the dependency of a highest cut-offfrequency on a collector current density between the transistor in thefirst embodiment according to the present invention and the transistorof the prior art;

[0037]FIG. 17 is a graph showing an example of a depth profile ofimpurity concentrations and Ge contents in a bipolar transistor using anexistent type selectively epitaxially grown SiGe base;

[0038]FIG. 18 shows an energy band structure in an active region of abipolar transistor using an existent type selectively epitaxially grownSiGe base;

[0039]FIG. 19 shows a preamplifier for an optical transmission system inthe eighth embodiment according to the present invention;

[0040]FIG. 20 is a constitutional view for a front-end module of anoptical transmission system in the ninth embodiment according to thepresent invention;

[0041]FIG. 21 is a constitutional view of an optical transmission systemin the tenth embodiment according to the present invention;

[0042]FIG. 22 shows an oscillator in a millimeter wave transmissionsystem in the eleventh embodiment according to the present invention;and

[0043]FIG. 23 shows an oscillator in a millimeter wave transmissionsystem in the twelfth embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Preferred embodiments of a bipolar transistor according to thepresent invention and optical communication systems and millimeter wavetransmission systems to which the bipolar transistor is applied are tobe explained below with respect to preferred embodiments from 1 to 12and related drawings to each of the embodiments.

[0045] Embodiment 1

[0046] The first embodiment according to the present invention is to beexplained with reference to FIG. 1 and to FIG. 4. FIG. 3 shows a crosssectional structure of a bipolar transistor using an SiGe alloy layerfor a base in the first embodiment according to the present invention.Cross sectional structures of bipolar transistors in other embodimentsaccording to the present invention are basically identical with thoseshown in this figure. In the drawing, are shown a p-type Si substrate 1,an n⁺-type Si layer 2, a low concentration n-type Si layer 3, an SiO₂film 4, an n⁺-type Si layer 5, an SiO₂ film 6, a non-doped polysiliconlayer 7, an Si₃N₄ film 8, a p-type polysilicon layer 9, an SiO₂ film 10,an n-type Si layer 11, a low concentration n-type SiGe film 12, a p-typeSiGe layer 13, a low concentration n-type SiGe layer 14, a non-dopedsingle crystalline Si layer 15, a p-type polycrystal SiGe layer 16, anSiO₂ film 17, an Si₃N₄ film 18, an n⁺-type polycrystal Si film 19, ann⁺-type Si layer 20, SiO₂ films 21, 22, an n-type polycrystal Si film23, and metal electrodes 24-26. Among the metal electrodes, 24 denotesan emitter electrode, 25 denotes a base electrode and 26 denotes acollector electrode.

[0047]FIG. 4 is a detailed cross sectional structure for a main portionof the SiGe base transistor shown in FIG. 3. Those reference numerals inthis figure are identical with those shown in FIG. 1. In this case, then⁺-type Si layer 2 serves as a collector, the p-type polysilicon layer 9serves as a base lead electrode, the n⁺-type polycrystal Si film 19 andthe n⁺-type Si layer 20 serves as an emitter.

[0048]FIG. 1 is a graph showing a depth profile of impurityconcentrations and Ge contents of the SiGe base transistor in the firstembodiment shown in FIG. 3 and FIG. 4, at a portion along a broken lineA in FIG. 4. The peak concentration of B in the base layer is 1×10²⁰cm⁻³. The Ge content is made uniform as about 20% in the B-doped regionand has a substantially trapezoidal profile with the maximum value atabout 30% in the emitter-base junction region and in the base-connectionjunction region adjacent therewith.

[0049]FIG. 2 shows an energy band structure in an active region of thetransistor in the first embodiment. As can be seen from the figure, anodge is formed in the valence band and at the edge of the base layer onthe side of the collector. On the other hand, no potential barrierhindering the electron transition is not present in the conduction band.

[0050] Then, a method of manufacturing an SiGe base transistor in thefirst embodiment explained with reference to FIG. 1 to FIG. 4 is to beexplained with reference to FIG. 5 and FIG. 6. The figures show crosssectional structures for main portions in main steps for manufacturingthe transistor. After forming an n⁺-type Si layer 2 as a buried layer, alow concentration n-type Si layer 3, and an SiO₂ film 4 for deviceisolation on a p-type Si substrate 1, an SiO₂ film 6, a non-dopedpolysilicon layer 7, an Si₃N₄ film 8, a p-type polysilicon layer 9 andan SiO₂ film 10 are deposited by way of a usual chemical vapordeposition (CVD) method (FIG. 5(a)). Then, an SiO₂ film 10 and a p-typepolysilicon layer 9 of a portion on the device forming region areremoved by usual photolithography and etching to form a window reachingthe Si₃N₄ film 8. Subsequently, an SiO₂ film 27 is deposited by the CVDmethod and, further, the SiO₂ film 27 for the portion other than theside wall of the window is removed by anisotropic dry etching. Then, anSi₃N₄ film 28 is deposited by the CVD method and, further, the Si₃N₄film 28 and the Si₃N₄ film 8 for the portion other than the side wall ofthe window are removed by anisotropic dry etching (FIG. 5(b)). Then, thenon-doped polysilicon layer 7 exposed to the bottom of the window isremoved by wet etching and, further, the SiO₂ film 6 exposed thereby tothe bottom of the window is removed by wet etching to expose the lowconcentration n-type Si layer 3 (FIG. 5 (c)). Then, a low concentrationn-type SiGe film 12, a p-type SiGe film 13, a low concentration n-typeSiGe film 14 and a non-doped single crystalline Si layer 15 areselectively grown epitaxially on the low concentration n-type Si layer 3at the bottom of the window successively by an ultra-high vacuum CVDmethod. Further, a p-type polycrystal SiGe layer 16 is grown from theexposed portion of the p-type polysilicon layer 9 simultaneously to joinwith the selective epitaxial layer (FIG. 6(d)). Then, after forming ann-type Si layer 11 by ion implanting P and heating, an SiO₂ film 17 andan Si₃N₄ film 18 are deposited by a usual CVD method and, further, theSi₃N₄ layer 18 and the SiO₂ film 17 for the portion other than the sidewall of the window are removed by anisotropic dry etching and wetetching. Then, a p-doped n⁺-type polycrystal Si layer 19 is depositedand heated by the usual CVD method to diffuse P into the non-dopedsingle crystal layer Si 15 to form an n⁺-type Si layer 20 in thenon-doped single crystalline Si layer 15. Further, an n⁺-typepolycrystal Si layer 19 for the portion other than the area just abovethe opening and the periphery thereof was removed selectively by theusual photolithography and etching (FIG. 6(e)). In the succeeding steps,the transistor shown in FIG. 1-FIG. 4 is completed by way of usual stepsof wiring and formation of intermetal layer.

[0051] A method of manufacturing bipolar transistors of otherembodiments according to the present invention is also identicalbasically as described above except for making the profiles differentbetween B and Ge in selective epitaxial growing.

[0052] In this embodiment, diffusion of B from the B-doped layer to aregion adjacent therewith can be suppressed. Therefore, at a B-dopedlayer width of 20 nm and a peak concentration of 1×10²⁰ cm⁻³ immediatelyafter the epitaxial growing for instance, the width of the B-dopedregion after emitter annealing is about 28 nm and it can be reduced toabout 70% relative to the width of about 40 mm in the prior art. As aresult, this can provide an effect capable of shortening the basetransit time of electron to about ½.

[0053] Further, in this embodiment, since a nodge is formed to a valenceband at the edge of the base layer on the side of the collector,increase in the effective base width due to the increase of thecollector current less occurs. Therefore, the collector current at whichthe cut-off frequency of the transistor becomes maximum is increased by50%. As a result, this can provide an effect of improving the maximumcut-off frequency to about 40%, in addition to the effect of shorteningthe base transit time mentioned above. FIG. 16 compares the dependenceof the maximum cut-off frequency on the collector current between thetransistor of this embodiment and the transistor of the prior art.

[0054] Embodiment 2

[0055] The graph shown in FIG. 7 shows a depth profile of impurityconcentrations and Ge contents in an SiGe base transistor in a thirdembodiment according to the present invention at a portion along thebroken line A in FIG. 4. The peak concentration of B in the base layeris 1×10²⁰ cm⁻³. The Ge content increases from about 15% to about 30%from the B-doped region to the base-collector junction region adjacenttherewith. On the other hand, it has a substantially trapezoidal profilewith the maximum value of about 30% in the emitter-base junction regionadjacent with the B-doped region.

[0056] According to this embodiment, since the Ge content has a grade inthe base layer, an electric field for accelerating electrons from theemitter to the collector is formed. Therefore, it can provide an effectcapable of shortening the base transit time of electrodes further byabout 20% compared with that of the transistor in the first embodiment.However, since the total amount of Ge contents is increased comparedwith that in the first embodiment, it causes a side-effect of increasingthe probability for the occurrence of crystal defects.

[0057] Embodiment 3

[0058] The graph of FIG. 8 shows a depth profile of impurityconcentrations and Ge contents in the SiGe base transistor in the thirdembodiment according to the present invention at a portion along thebroken line in FIG. 4. The B peak concentration in the base layer is1×10²⁰ cm⁻³. The Ge content increases from about 15% to about 20% in thedirection from the emitter to the collector in the B-doped region. Onthe other hand, it forms a substantially trapezoidal profile with themaximum value of about 30% in the emitter-base junction region and thebase-collector junction region adjacent with the B-doped region. Thegrade of the Ge content on both ends of the B-doped region is greaterthan the grade in the B-doped region.

[0059] According to this embodiment, since the Ge content has a grade inthe base layer like that the second embodiment, the base transit time ofelectrons can be made identical with that of the transistor in thesecond embodiment. Further, since the total amount of the Ge contents isless than that in the second embodiment, it has an effect capable ofreducing the probability for the occurrence of crystal defects to alevel substantially equal with that in the first embodiment.

[0060] Further, in this embodiment, since a nodge is formed in thevalence band at the edge of the base layer on the side of the collectorlike that in the first embodiment, increase of the effective width ofthe base due to the increase of the collector current less occurs atwhich the cut-off frequency of the transistor becomes maximum isincreased by 25% and, as a result, it provides an effect of improvingthe cut-off frequency by 8%.

[0061] Embodiment 4

[0062] The fourth embodiment according to the present invention is to beexplained with reference to FIG. 9 and FIG. 10. FIG. 9 is a graphshowing the depth profile of impurity concentrations and Ge contents inthe SiGe base transistor in the fourth embodiment according to thepresent invention at a portion along the broken line A shown in FIG. 4.The B peak concentration in the base layer is 1×10²⁰ cm⁻³. The Gecontent is uniform as about 20% in the B-doped region and theemitter-base junction region adjacent therewith. On the other hand, ithas a substantially trapezoidal profile with the maximum value of about35% in the base-collector junction region adjacent with the B-dopedregion. FIG. 10 shows an energy band structure in an active region ofthe transistor in the fourth embodiment. As can be seen from the figure,a nodge is formed in the valence band at the edge of the base on theside of the collector. Further, no potential barrier inhibiting theelectron transition are not present in the conduction band of thistransistor.

[0063] According to this embodiment, the Ge content is decreased in theemitter-base junction region and, instead, the Ge content is increasedin the base-collector region compared with the transistor in the firstembodiment. Therefore, the B diffusion speed in the base-collectorjunction in which B tends to diffuse more can be lowered withoutincreasing the total amount of the Ge contents. As a result, the widthof the B-doped region can be decreased by about 7%, that is, to 26 mmcompared with the first embodiment. As a result, it can provide aneffect capable of reducing the base transit time of electrons by about14%.

[0064] Embodiment 5

[0065] The graph of FIG. 11 shows the depth profile of the impurityconcentrations and Ge contents in the SiGe base transistor in the fifthembodiment according to the present invention at a portion along thebroken line A shown in FIG. 4. The B peak concentration is 1×10²⁰ cm⁻³in the base layer. The Ge content increases from about 20% to about 28%in the direction from the emitter to the collector in the B-dopedregion. On the other hand, a region with a uniform content at about 20%is present in the emitter-base junction region adjacent with the B-dopedregion. Further, it has a substantially trapezoidal profile with themaximum value of about 35% in the base-collector junction region. Thegrade of the Ge content at the edge of the B-doped region on the side ofthe collector is greater than the grade in the B-doped region.

[0066] According to this embodiment, since the Ge content has a grade inthe base layer, an electric field for accelerating electrons from theemitter to the collector is formed. Accordingly, it can provide aneffect capable of decreasing the base transit time of electron furtherby about 20% compared with the transistor in the fourth embodiment.

[0067] Embodiment 6

[0068] The sixth embodiment according to the present invention is to beexplained with reference to FIG. 12 and FIG. 13. FIG. 12 is a graphshowing the depth profile of impurity concentrations and Ge contents inthe SiGe base transistor in the sixth embodiment according to thepresent invention at a portion along the broken line A shown in FIG. 4.The B peak concentration in the base layer 1×10²⁰ cm⁻³. The Ge contentis uniform at about 20% in the B-doped region and decreases abruptlyfrom the edge of the B-doped region to the emitter-collector junctionregion adjacent therewith. On the other hand, it has a substantiallytrapezoidal profile with the maximum value of about 40% in thebase-collector junction region adjacent with the B-doped region. FIG. 13shows an energy band structure in an active region of the transistor inthe sixth embodiment. As can be seen from the figure, a nodge is formedin the valence band at the edge of the base on the side of thecollector. Further, no potential barrier inhibiting the electrontransition is present in the conduction band of this transistor.

[0069] According to this embodiment, the Ge content in thebase-collector junction region is increased more instead of decreasingthe Ge content in the emitter-base junction region to 0 compared withthe transistor in the fourth embodiment. Accordingly, the B diffusionspeed in the base-collector junction can be further lowered withoutincreasing the total amount of Ge contents. As a result, this canprovide an effect capable of making the width of the B-doped region andthe base transit time of electrons substantially equal with those in thefourth embodiment.

[0070] Embodiment 7

[0071] The graph of FIG. 14 shows the depth profile of the impurityconcentrations and the Ge contents in the SiGe base transistor in theseventh embodiment according to the present invention at a portion alongthe broken line A shown in FIG. 4. The B peak concentration is 1×10²⁰cm⁻³ in the base layer. The Ge content increases from about 0% to about28% in the direction from the emitter to the collector in the B-dopedregion. On the other hand, it has a substantially trapezoidal profilewith the maximum value of about 40% in the base-collector junctionregion. The grade of the Ge content at the edge of the B-doped region onthe side of the collector is larger than the grade in the B-dopedregion.

[0072] According to this embodiment, since the Ge content has a grade inthe base layer, an electric field for accelerating electrons from theemitter to the collector is formed. Accordingly, it can provide aneffect capable of shortening the base transit time of electrons byfurther about 20% compared with the transistor in the sixth embodiment.

[0073] Embodiment 8

[0074]FIG. 19 shows a diagram for a preamplifier circuit in an opticaltransmission system illustrating an eighth embodiment according to thepresent invention. This embodiment is an example of using the SiGe basebipolar transistor in the previous embodiments of a transistor 301 foramplification, and transistors 302 and 303 for a buffer. This is acircuit of amplifying an input from a photodiode 306, and outputting thesame from an output buffer 307 by way of an amplifier comprisingtransistors 301, 302 and 303 and registers 304 and 305. This circuit hasa frequency band higher than 40 GHz by the use of the SiGe base bipolartransistor in the embodiment described above.

[0075] The frequency band of this circuit can be made to 40 GHz orhigher also by using a transistor comprising a compound semiconductorsuch as GaAs instead of the SiGe base bipolar transistor. However, thisembodiment using the SiGe base bipolar transistor has an advantageousfeature compared with the case of using the compound semiconductordevice in that the cost is reduced since the material for the substrateis inexpensive, consumes less electric power since the shrink of thedevice manufactured by using the well established Si fabricationprocesses is easy and the reliability is high since this is an Si seriesdevice.

[0076] Embodiment 9

[0077]FIG. 20 shows a front-end module including a photodiode and apreamplifier in an optical receiving module showing a ninth embodimentaccording to the present invention. This embodiment is an example ofapplying the preamplifier comprising the SiGe base bipolar transistor ofthe embodiments described previously to a front end module. An opticalsignal inputted from an optical fiber 401 is focused by a lens 402 andconverted by a photodiode IC 403 into an electric signal. The electricsignal is amplified by a preamplifier IC 404 through a wiring 405 on asubstrate 407 and then outputted from an output node 406.

[0078] Embodiment 10

[0079]FIG. 21 is a constitutional view for an optical transmissionsystem illustrating a tenth embodiment according to the presentinvention. This embodiment is an example of applying the SiGe basebipolar transistor of the previous embodiments to the circuits in bothof the transmission systems of an optical transmission module 513 fortransmitting data at a super-high speed and an optical receiver module514 for receiving the data.

[0080] The module comprises a multiplexer 501 for processing electricsignals 510 to be transmitted, a semiconductor laser 503, asemiconductor laser driving analog circuit 502 for driving asemiconductor laser 503, a preamplifier 505 for amplifying a receivedelectric signal 512 prepared by conversion of transmitted optical signal511 by a photodiode 504, as well as each of analog circuits of anautomatic-gain-control amplifier 506, a clock extraction circuit 507 anda decision circuit 508, and a demultiplexer 509 as a digital circuit. Inthis embodiment, transistors in the multiplexer 501, the analog circuit502 for the semiconductor laser driver, the preamplifier 505, theautomatic-gain-control amplifier 506, the clock extraction circuit 507,the decision circuit 508 and the demultiplexer 509 are SiGe base bipolartransistors in the previous embodiments. Since the SiGe base bipolartransistor has a cut-off frequency and a maximum cut-off frequency ashigh as 200 GHz, this transmission system can transmit/receive signalsat a large capacity of 40 G bits or more per one sec at a super-highspeed. This embodiment also has a similar feature as that in the eighthembodiment.

[0081] Embodiment 11

[0082]FIG. 22 shows an oscillator circuit in a millimeter wavetransmission system illustrating an eleventh embodiment according to thepresent invention. This circuit comprises a transistor 601, a varactordiode 602, ¼ λ stubs 603 and 604, feedback stubs 607 and 608 andtransmission lines 609 and 610 as a matching circuit. This embodiment isan example of using the SiGe base bipolar transistor in the previousembodiments to the transistor 601. This is a circuit for supplyingmillimeter wave band frequency signals to a transmission/receiver mixer.This circuit has a band frequency of 60 GHz or higher by using the SiGebase bipolar transistor of the embodiments described previously.

[0083] The band frequency of this circuit can be made to 60 GHz orhigher also by using a transistor comprising a compound semiconductorsuch as GaAs instead of the SiGe base bipolar transistor. However, thisembodiment using the SiGe base bipolar transistor has an advantageousfeature compared with the case of using the compound semiconductordevice in that the cost is reduced since the material for the substrateis inexpensive, it consumes less electric power since the shrink of thedevice is easy by using the well established Si fabrication processes,and the reliability is high since this is an Si device.

[0084] Embodiment 12

[0085]FIG. 23 is a constitutional view for a millimeter wavetransmission system illustrating a twelfth embodiment according to thepresent invention. This embodiment is an example of applying the SiGebase bipolar transistor in the previous embodiments to the circuit inthe millimeter wave transmission system.

[0086] This millimeter wave transmission system comprises a transmittingoscillator 701, a transmitting mixer 702 for receiving a transmissionelectric signal and stepping up the frequency, filters 703 and 710, apower amplifier 704 for receiving the output from the transmittingmixer, a transmitter/receiver switch 705, a receiving antenna 706 ormillimeter wave, a low noise amplifier 707 for amplifying the receivedelectric signal from the antenna, a receiving oscillator 708, and areceiving mixer 709 for receiving the output from the low noiseamplifier for stepping-down the frequency. In this embodiment,transistors in the transmitting oscillator 701, the transmitting mixer702, the power amplifier 704, the low noise amplifier 707, the receivingoscillator 708, and the receiving mixer 709 are SiGe base bipolartransistors in the previous embodiments. Since the cut-off frequency andthe maximum cut-off frequency are as high as 200 GHz, thistransmitting/receiving system can transmit/receive millimeter wavesignals at a frequency of 60 GHz or higher. This embodiment also has thesame advantageous features as the eighth embodiment.

[0087] According to the present invention, the base diffusion speed inthe emitter-base junction region and the base-collector junction regionadjacent with the B-doped layer of the base can be lowered. As a result,at the width of the B-doped layer for the base of 20 nm and the peakconcentration of 1×10²⁰ cm⁻³ immediately after epitaxial growing, thewidth of the B-doped region after emitter annealing is about 26 nm to 28nm, which can be reduced by about 65% to 70% relative to about 40 mmwidth in the prior art. As a result, base transit time of electrons canbe decreased to about 42%-50%.

[0088] Further, according to the present invention, since a nodge isformed in the balance electron band at the edge of the base on the sideof the collector, increase of the effective base width due to theincrease of the collector current occurs less. Accordingly, thecollector current at which the cut-off frequency of the transistorreaches maximum can be increased by about 50% compared with the priorart. As a result, it can provide an effect of improving the maximumcut-off frequency by 40% to 70%, coupled with the effect of shorteningthe base transit time.

What is claimed is:
 1. A bipolar transistor using a B-doped Si and Gealloy for a base in which the maximum value of a Ge content in anemitter-base junction depletion region and a base-collector junctiondepletion region is greater than an average value in a base layer.
 2. Abipolar transistor according to claim 1, wherein a grade of Ge contentin a region in which the grade of Ge content increases from the vicinityof an edge of a base layer on a collector side to the collector isgreater than an average grade of Ge content in the base layer.
 3. Abipolar transistor using a B-doped Si and Ge alloy for a base in whichthe maximum value of a Ge content in a base-collector junction depletionregion is greater than an average value in a base layer, wherein thegrade of Ge content in a region in which the grade of Ge contentincreases from the vicinity of an edge of the base layer on a collectorside to the collector is greater than an average grade of Ge content inthe base layer.
 4. A bipolar transistor using a B-doped Si and Ge alloyfor a base in which the maximum value of a Ge content in abase-collector junction depletion region is greater than an averagevalue in a base layer, wherein the transistor has a region of a constantGe content in a region from the edge of the base layer on the side of anemitter to an emitter-base junction.
 5. A bipolar transistor accordingto claim 4, wherein the grade of Ge content in a region where the gradeof Ge content increases in a region from the vicinity of the edge of thebase layer on a collector side to the collector is greater than anaverage grade of Ge content in the base layer.
 6. An opticaltransmitter/receiver system comprising an optical receiver system havinga photodetector for receiving an optical signal and outputting anelectric signal, a first amplifier for receiving the electric signalfrom the photodetector, a second amplifier for receiving the output fromthe first amplifier, a decision circuit for converting the output fromthe second amplifier into a digital signal in synchronization with apredetermined clock signal and a demultiplexer for separating andconverting the digital signal, as well as a photo-transmitter systemhaving a multiplexer for synthesizing multiple digital signals, asemiconductor laser and a semiconductor laser driver for driving thesemiconductor laser, wherein at least one of transistors in the firstand the second amplifiers, the decision circuit, the digital signaldemultiplexer, the multiplexer and the semiconductor laser driver isconstituted with the SiGe base bipolar transistor according to claim 1,2, 3, 4 or
 5. 7. An millimeter wave transmission system comprising: areceiving antenna for millimeter wave, a first amplifier for amplifyinga received electric signal from the antenna, a receiving mixer forreceiving the output from the first amplifier and stepping-down thefrequency, a first oscillator, a transmitting mixer for receiving atransmission electric signal and stepping-up the frequency, a secondoscillator, and a second amplifier for receiving the output from thetransmitting mixer and amplifying the power, wherein at least one of thetransistors in the first and the second amplifiers, the first and thesecond oscillators, and receiving mixer and the transmitting mixer isconstituted with the SiGe base bipolar transistor according to claim 1,2, 3, 4 or 5.